Publications and other reference materials referred to herein, including reference cited therein, are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
In the past decade fiber lasers have become exceedingly widespread and are replacing bulk solid state lasers in diverse applications. The ability to efficiently absorb the pump light by use of double clad fibers, the high gain achieved by long interaction lengths, the favorable heat dissipation configuration, the excellent beam quality dictated by the single mode waveguide, and the ease of alignment, all together make the fiber laser very attractive, combining high output power and excellent beam quality in a reliable and relatively low cost configuration. However, with high CW powers or high peak power pulses, where the intensity in the core is very high, deleterious nonlinear effects and material damage limit the fiber laser performance.
Two main approaches have been developed to overcome this problem. The more straightforward and common approach is using step index fibers with very small refractive index difference Δn between the core and the cladding (often referred to as Large Mode Area (LMA) fibers). These fibers have large cores supporting large transverse mode operation and thus the intensity in the core is reduced. Due to the inability to fabricate fibers with very small Δn this approach is currently limited to modes with less than 30 μm mode field diameter. Furthermore, these fibers are not strictly single mode and coiling should be used to achieve truly single mode operation. This coiling often results in smaller and asymmetric modes and reduced efficiency[1], [2].
The second approach is the use of large single mode, double clad, photonic crystal fibers (PCFs). Here the large single mode cores are achieved by introducing very small air holes in the silica, creating a very small effective Δn. Mode field diameters of 30 μm have been achieved with flexible PCFs and 80 μm with rigid rod-type PCFs. The main disadvantage of this approach is the lack of an “all-fiber” configuration, which results from the difficulty to splice PCFs to side pump couplers, to fiber bragg gratings (FBG) and to end caps. The need for free space light coupling reduces dramatically the robustness of the system [3], [4]. Moreover, the fabrication of PCFs is relatively complicated and costly, with only a few companies offering them commercially.
It is a purpose of the present invention to provide an active double-clad fiber that guides only a single transverse mode that has large overlap with a doped active cladding and has very large mode area with good modal discrimination.
It is another purpose of the invention to provide an active double-clad fiber that is simple to fabricate compared to PCFs.
It is another purpose of the invention to provide an active double-clad fiber that can be spliced and used in an “all-fiber” configuration with improved performance compared to standard active LMA and PCF fibers.
Further purposes and advantages of this invention will appear as the description proceeds.